It has been long recognized in the prior art that when conductivity-determining impurities such as phosphorus or boron which have atom radii substantially smaller than that of silicon are introduced into a silicon substrate, misfit dislocations in the silicon will occur which interfere with the eventual performance of the integrated circuit being fabricated. The article, "Strain Compensation in Silicon by Diffused Impurities", T. H. Yeh et al, J. Electrochem. Soc: Solid State Science, January 1969, pp. 73 - 77, which deals with the introduction of impurities by thermal diffusion sets forth that when diffusing boron or phosphorus atoms which have tetrahedral covalent radii or atomic radii which differ considerably from that of silicon, such atoms represent misfits in the silicon crystal lattice structure, and such misfits create strains which result in misfit dislocations. J. R. Carruthers et al, in "X-Ray Investigation of the Perfection of Silicon", Journal of Applied Physics, Vol. 34, No. 11, November 1963, pp. 3389 - 3393, define such a misfit ratio .GAMMA. = (r.sub.i /r.sub.Si) where r.sub.i is the tetrahedral covalent radius of the impurity atom and r.sub.Si is the atomic radius of silicon. Phosphorus has a misfit ratio of 0.932 while boron has an even more severe misfit ratio of 0.746. However, they indicate that arsenic, As, has a misfit ratio .GAMMA. approaching 1.000. Because this ratio constitutes a near perfect fit, As will not be expected to generate any strains when introduced into silicon, Ibid, pp. 3392 - 3393.
The art has further recognized as indicated by Yeh et al, that dislocations in silicon caused by either the diffused boron or phosphorus misfits may be compensated for through the introduction into the diffused silicon substrate regions of inert or nonconductivity-determining ions such as tin or germanium. In view of the disclosure in the Carruthers et al article that both boron and phosphorus have atomic radii substantially smaller than that of silicon, the above Yel et al article goes on to teach that the tin or germanium atoms which have atomic radii larger than that of silicon provide misfits in the opposite sense to that of the phosphorus or boron dopants and, thus, balance the crystal lattice structure, and thereby minimize lattice strains and the resulting misfit dislocations.
With the progress of the art toward the use of ion implantation for the introduction of conductivity-determining impurities into silicon structures. N. Yoshihiro et al, Ion Implantation in Semiconductors, Proceedings of the 4th International Conference on Ion Implantation, edited by S. Namba, Plenum Press, 1975, pp. 571 - 576, recognized that the same misfit dislocation problem would occur in the formation of substrate regions by the introduction of the phosphorus atoms into the silicon by ion implantation. Yoshihiro et al, further found that the misfit problem created by the ion implantation of phosphorus could, likewise, be solved by the implantation of the germanium along with the phosphorus in a double ion implantation step.
Thus, while the prior art recognized that the introduction of germanium minimized misfit dislocation problems resulting from either boron or phosphorus misfits in silicon, it has not suggested any reason or advantage for the introduction of germanium into regions doped with arsenic impurities in silicon substrates. This was apparently due to the understanding in the art that as set forth in Carruthers et al since arsenic has an atomic radius very close to that of silicon and thus a misfit ratio approaching one indicating an absence of misfits, it would serve no known purpose to incorporate germanium into an arsenic-doped silicon.